1. Field of the invention
The present invention relates to a process for fabricating mirrors, and more particularly to a process for fabricating micro-mirrors on a silicon substrate.
2. Description of prior art
Nowadays, silicon-based optical systems or components dominate the development of micro-optics technology and are used indifferent ways, such as optical pick-up head apparatus, laser collimating lens and WDM for fiber-optical communication disclosed, for example, in U.S. Pat. No. 5,500,910.
Referring to FIG. 1, U.S. Pat. No. 5,293,038 has disclosed the structure of an optical pick-up head apparatus, which comprises a laser source 11, a first semiconductor substrate 12, a second semiconductor substrate 13, a holographic optics 14, a photo-detector 15, mirrors 17 and 18, an optical lens 19 and an optical disk 20. In this structure, optical mirrors are fabricated on (100) silicon. 45xc2x0 optical mirrors 17 are formed by etching the silicon at an angle of 9xc2x0 from the {110} surface. The mirrors are coated with Au to increase the reflectance. All of the components of an optical pick-up head apparatus are integrated into a single chip to reduce its weight and volume. Therefore by using the prior-art structure the size of the pick-up head apparatus can be precisely controlled, the pick-up device is easy to mass-produce, and the manufacturing cost can be reduced. However, since the optical pick-up head is a micro-optics formed on silicon, the flatness of silicon surface must be controlled within xcex/4. To form micromirrors in a dimension of a few hundred micrometers to several ten micrometers is difficult when using conventional polishing. Accordingly, current research is primarily directed toward discovering how to improve the flatness of micromirror surfaces to reduce the scattering effect on the laser and to raise the S/N ratio.
An object of the present invention is to provide a fabrication process of micromirrors on a silicon substrate to improve the flatness of micromirrors; thereby reducing the scattering effect on the laser, and increasing the S/N ratio.
The present invention is implemented on silicon. A SiO2 layer is first coated on the surfaces of micromirrors after forming the micromirrors by etching the silicon. Then, a CO2 laser with a wavelength of 10.6 xcexcm is applied to illuminate the silicon substrate. The silicon substrate is transparent to the 10.6 xcexcm CO2 laser. That is the silicon substrate will not absorb the CO2 laser light. On the contrary, SiO2 is opaque to a 10.6 xcexcm CO2 laser. Therefore, a 10.6 xcexcm CO2 laser can be utilized to perform annealing for SiO2 to improve the surface flatness. In this way, the roughness can be controlled under xcex/4.
Another implementation of this invention is to coat a layer of gold on the silicon to form a eutectic structure, which is then heated to increase the surface flatness.
After the flatness of the micro-mirror""s surface has been improved, a layer of gold is coated to increase the reflectance of the micro-mirror.